Hi, I want to make my own battery pack combined from two 18650 cells, connected as 2S. So I need a PCB - I found some with over discharge voltage 2.4V and others with 2.8V. Which one should I use? As 18650 cells have usually end voltage 2.5V, I am not sure. On the one side I want to get as much power as I can (I will not over discharge often and will charge with low current), on the other side can be 2.4V dangerous for the cells?
Remember voltage sag; it’ll depend on how much your use loads the cells, as the voltage drops under load — and how good the cells are, and how old they are.
So under load, the protection operates — then when you revive the cell it’s somewhat less discharged than that cutoff amount, when tested with no load — so less degraded, probably.
(this is why a trickle discharge can kill a cell, if the protection level is too low, and voltage never sags, the cell will be truly drained to that voltage.)
Caution, I know nothing about this, I’m just some guy on the Internet.
Do not blow yourself up by relying on anything you read under my name.
2.4V is what is used for any new li-ion cell today.
When you say “need a PCB” - what exactly do you mean? Are you going to use cylindrical protective PCBs from protected cells and you shrink-wrap them yourself (= 1 PCB per cell)? Or are you creating custom Lithium Packs and, as stated, are going to build a 2 LiIon series pack with ONE standard charge/discharge/protection PCB?
If you are planning to put two Lithium cells in series and if you are going to charge/discharge/protect them with 1 PCB (let’s say all the standard lithium boards on AE/eBay/GB/BG) you need to factor in two cells and a different LVP value.
edit: same applies for charging 2S lithium cells. different termination voltage than the regular boards have.
thanks for answers
I want use 1 PCB, (I heard it’s not good to use protected cells in series and it’s better use 1 PCB per pack), with nominal output voltage 7.4V
So I assume this will be ok for me, right?
that one looks good for 2s lithium cells. there isn’t much detail on the product page but it should work.
Interesting that it has 5A discharge!
Go for 2.8 Volts.
There’s not much left in a cell below 3.0 Volts anyway and cells don’t like to go there either.
With 2.8V LVP in a 2S pack you’d kill the cells.
You said you had to choose between 2.4 and 2.8 Volts
You need to protect each cell separately.
But this: Page Not Found - Aliexpress.com does that already.